Source/drain feature separation structure

ABSTRACT

A semiconductor device according to the present disclosure includes a first source/drain feature, a second source/drain feature, a third source/drain feature, a first dummy fin disposed between the first source/drain feature and the second source/drain feature along a direction to isolate the first source/drain feature from the second source/drain feature, and a second dummy fin disposed between the second source/drain feature and the third source/drain feature along the direction to isolate the second source/drain feature from the third source/drain feature. The first dummy fin includes an outer dielectric layer, an inner dielectric layer over the outer dielectric layer, and a first capping layer disposed over the outer dielectric layer and the inner dielectric layer. The second dummy fin includes a base portion and a second capping layer disposed over the base portion.

BACKGROUND

The semiconductor integrated circuit (IC) industry has experiencedexponential growth. Technological advances in IC materials and designhave produced generations of ICs where each generation has smaller andmore complex circuits than the previous generation. In the course of ICevolution, functional density (i.e., the number of interconnecteddevices per chip area) has generally increased while geometry size(i.e., the smallest component (or line) that can be created using afabrication process) has decreased. This scaling down process generallyprovides benefits by increasing production efficiency and loweringassociated costs. Such scaling down has also increased the complexity ofprocessing and manufacturing ICs.

For example, as integrated circuit (IC) technologies progress towardssmaller technology nodes, multi-gate devices have been introduced toimprove gate control by increasing gate-channel coupling, reducingoff-state current, and reducing short-channel effects (SCEs). Amulti-gate device generally refers to a device having a gate structure,or portion thereof, disposed over more than one side of a channelregion. Fin-like field effect transistors (FinFETs) andmulti-bridge-channel (MBC) transistors are examples of multi-gatedevices that have become popular and promising candidates for highperformance and low leakage applications. A FinFET has an elevatedchannel wrapped by a gate on more than one side (for example, the gatewraps a top and sidewalls of a “fin” of semiconductor material extendingfrom a substrate). An MBC transistor has a gate structure that canextend, partially or fully, around a channel region to provide access tothe channel region on two or more sides. Because its gate structuresurrounds the channel regions, an MBC transistor may also be referred toas a surrounding gate transistor (SGT) or a gate-all-around (GAA)transistor. The channel region of an MBC transistor may be formed fromnanowires, nanosheets, or other nanostructures and for that reasons, anMBC transistor may also be referred to as a nanowire transistor or ananosheet transistor.

IC devices may include repeating physical design blocks that arereferred to as standard cells. Such standard cells may include logicgates, such as NAND, NOR, XNOR, XOR, AND, OR, INVERTER standard cells,or memory bits, such as SRAM cells. One way to achieve smaller geometricsizes is to reduce the dimensions of a standard cell. Because standardcells are repeated multiple times, a dimensional reduction in a standardcell may translate into substantial reduction in size. A standard cellmay include multiple active regions (such as multiple fin structures ormultiple stacks of bridge-like channel members) that are interleaved bymultiple dummy fins that function to isolate source/drain features ofneighboring devices. Dummy fins take up space and may pose challengeswhen dimensions of standard cells shrink. Therefore, while conventionaldummy fins and methods of forming the same may be generally adequate fortheir intended purposes, they are not satisfactory in all aspects.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure is best understood from the following detaileddescription when read with the accompanying figures. It is emphasizedthat, in accordance with the standard practice in the industry, variousfeatures are not drawn to scale and are used for illustration purposesonly. In fact, the dimensions of the various features may be arbitrarilyincreased or reduced for clarity of discussion.

FIG. 1 illustrates a flowchart of a method for forming a semiconductordevice, according to one or more aspects of the present disclosure.

FIGS. 2-12 illustrate fragmentary cross-sectional views of a workpieceduring a fabrication process according to the method of FIG. 1 ,according to one or more aspects of the present disclosure.

FIG. 13 illustrates a fragmentary top view of an example semiconductordevice fabricated according to the method of FIG. 1 , according to oneor more aspects of the present disclosure.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly. Still further, when anumber or a range of numbers is described with “about,” “approximate,”and the like, the term is intended to encompass numbers that are within+/−10% of the number described, unless otherwise specified. For example,the term “about 5 nm” encompasses the dimension range from 4.5 nm to 5.5nm.

The present disclosure is generally related to source/drain separationstructures and fabrication methods, and more particularly to formingdifferent dummy fins to achieve different spacings between activeregions for either dimensional reduction or performance improvement.

As described above, MBC transistors may also be referred to as SGTs, GAAtransistors, nanosheet transistors, or nanowire transistors. They can beeither n-type or p-type. A standard cell may include multiple MBCtransistors that are formed from fin-shaped structures. Each of thesefin-shaped structures includes a plurality of channel layers interleavedby a plurality of sacrificial layers. In some conventional technologies,the fin-shaped structures are parallel to one another and are equallyspaced and dummy fins are inserted between fin-shaped structures. Thedummy fins function to separate source/drain features of neighboringdevices. When fin-shaped structures are not equally spaced, widths ofthe dummy fins may vary and etch loading may result into uneven heightsand structures of dummy fins, when may lead to merging of source/drainfeatures and device failures.

The present disclosure provides a process for forming different dummyfins of varying widths and structures. Such different dummy fins mayinclude a first dummy fin and a second dummy fin that is narrower thanthe first dummy fin. Due to the width difference, the first dummy finand the second dummy fin have different structures that may etchdifferently in an etch back process. The present disclosure provides aprocess to ensure even heights of the first dummy fin and the seconddummy fin to satisfactorily separate neighboring source/drain features.The process and structure of the present disclosure enables reduction ofstandard cell sizes or improvement of performance of a standard cell.

The various aspects of the present disclosure will now be described inmore detail with reference to the figures. FIG. 1 illustrates aflowchart of a method 100 of forming a semiconductor device from aworkpiece according to one or more aspects of the present disclosure.Method 100 is merely an example and is not intended to limit the presentdisclosure to what is explicitly illustrated in method 100. Additionalsteps may be provided before, during and after method 100, and somesteps described can be replaced, eliminated, or moved around foradditional embodiments of the methods. Not all steps are describedherein in detail for reasons of simplicity. Method 100 is describedbelow in conjunction with FIGS. 2-12 , which illustrate fragmentarycross-sectional views of the workpiece at different stages offabrication according to embodiments of method 100.

Referring to FIGS. 1 and 2 , method 100 includes a block 102 where aworkpiece 200 is received. Because a semiconductor device will be formedfrom the workpiece 200, the workpiece 200 may be referred to as asemiconductor device 200 as the context requires. As shown in FIG. 2 ,the workpiece 200 includes a substrate 202 and a stack 204 disposed onthe substrate 202. In one embodiment, the substrate 202 may be a silicon(Si) substrate. In some other embodiments, the substrate 202 may includeother semiconductors such as germanium (Ge), silicon germanium (SiGe),or a III-V semiconductor material. Example III-V semiconductor materialsmay include gallium arsenide (GaAs), indium phosphide (InP), galliumphosphide (GaP), gallium nitride (GaN), gallium arsenide phosphide(GaAsP), aluminum indium arsenide (AlInAs), aluminum gallium arsenide(AlGaAs), gallium indium phosphide (GaInP), and indium gallium arsenide(InGaAs). The substrate 202 may also include an insulating layer, suchas a silicon oxide layer, to have a silicon-on-insulator (SOI) structureor a germanium-on-insulator (GOI) structure. In some embodiments, thesubstrate 202 may include one or more well regions, such as n-type wellregions doped with an n-type dopant (i.e., phosphorus (P) or arsenic(As)) or p-type well regions doped with a p-type dopant (i.e., boron(B)), for forming different types of devices. The doping the n-typewells and the p-type wells may be formed using ion implantation orthermal diffusion.

Referring still to FIG. 2 , the stack 104 may include a plurality ofchannel layers 208 interleaved by a plurality of sacrificial layers 206.The channel layers 208 and the sacrificial layers 206 may have differentsemiconductor compositions. In some implementations, the channel layers208 are formed of silicon (Si) and sacrificial layers 206 are formed ofsilicon germanium (SiGe). In these implementations, the additionalgermanium content in the sacrificial layers 206 allow selective removalor recess of the sacrificial layers 206 without substantial damages tothe channel layers 208. In some embodiments, the sacrificial layers 206and channel layers 208 may be deposited using an epitaxial process. Thestack 204 may be epitaxially deposited using CVD deposition techniques(e.g., vapor-phase epitaxy (VPE) and/or ultra-high vacuum CVD(UHV-CVD)), molecular beam epitaxy (MBE), and/or other suitableprocesses. The sacrificial layers 206 and the channel layers 208 aredeposited alternatingly, one-after-another, to form the stack 204. It isnoted that four (4) layers of the sacrificial layers 206 and three (3)layers of the channel layers 208 are alternately and vertically arrangedas illustrated in FIG. 2 , which are for illustrative purposes only andnot intended to be limiting beyond what is specifically recited in theclaims. The number of layers depends on the desired number of channelsmembers for the semiconductor device 200. In some embodiments, thenumber of the channel layers 208 is between 2 and 10. For patterningpurposes, the workpiece 200 may also include a hard mask layer 210 overthe stack 204. The hard mask layer 210 may be a single layer or amultilayer. In one example, the hard mask layer 210 includes a siliconoxide layer and a silicon nitride layer.

Referring to FIGS. 1 and 3 , method 100 includes a block 104 where afirst fin-shaped structure 212-1, a second fin-shaped structure 212-2, athird fin-shaped structure 212-3, a fourth fin-shaped structure 212-4,and a fifth fin-shaped structure 212-5 are formed. For ease ofreference, the first fin-shaped structure 212-1, the second fin-shapedstructure 212-2, the third fin-shaped structure 212-3, the fourthfin-shaped structure 212-4, and the fifth fin-shaped structure 212-5 maybe collectively referred to as fin-shaped structures 212. As shown inFIG. 3 , the fin-shaped structure 212 are formed from the stack 204 anda portion of the substrate 202. In some embodiments, at block 104, thestack 204 and the substrate 202 are patterned to form the fin-shapedstructures 212. The fin-shaped structures 212 extend vertically alongthe Z direction from the substrate 202. Each of the fin-shapedstructures 212 includes a base portion 12B formed from the substrate 202and a stack portion 12S formed from the stack 204. The fin-shapedstructures 212 may be patterned using suitable processes includingdouble-patterning or multi-patterning processes. Generally,double-patterning or multi-patterning processes combine photolithographyand self-aligned processes, allowing patterns to be created that have,for example, pitches smaller than what is otherwise obtainable using asingle, direct photolithography process. For example, in one embodiment,a material layer is formed over a substrate and patterned using aphotolithography process. Spacers are formed alongside the patternedmaterial layer using a self-aligned process. The material layer is thenremoved, and the remaining spacers, or mandrels, may then be used topattern the fin-shaped structures 212 by etching the stack 204 and thesubstrate 202. The etching process can include dry etching, wet etching,reactive ion etching (RIE), and/or other suitable processes.

In some embodiments presented in FIG. 3 , the fin-shaped structures 212are differently spaced. The first fin-shaped structure 212-1 is spacedapart from the second fin-shaped structure 212-2 by a first spacing S1.The second fin-shaped structure 212-2 is spaced apart from the thirdfin-shaped structure 212-3 by a second spacing S2. The third fin-shapedstructure 212-3 is spaced apart from the fourth fin-shaped structure212-4 by the first spacing S1. The fourth fin-shaped structure 212-4 isspaced apart from the fifth fin-shaped structure 212-5 by the firstspacing S1. In the illustrated example, the second spacing S2 is smallerthan the first spacing S1. According to the present disclosure, thesmaller second spacing S2 may be implemented for at least two reasons.In the context of standard cells, the smaller second spacing S2 mayreduce a width of a standard cell as compared to another standard cellthat includes uniform first spacings S1. When a width of a standard cellis fixed, the smaller second spacing S2 may translate into larger firstspacings S1 or wider fin-shaped structures 212 for improvement of deviceperformance. In some instances, the first spacing S1 may be betweenabout 15 nm and about 40 nm and the second spacing S2 may be betweenabout 5 nm and about 40 nm.

Referring to FIGS. 1 and 3 , method 100 includes a block 106 where anisolation feature 203 is formed. After the fin-shaped structures 212 areformed, the isolation feature 203 is formed between neighboringfin-shaped structures 212. The isolation feature 203 may also bereferred to as a shallow trench isolation (STI) feature 203. In anexample process, a dielectric layer is first deposited over theworkpiece 200, filling the trenches between fin-shaped structures 212with the dielectric material. In some embodiments, the dielectric layermay include silicon oxide, silicon nitride, silicon oxynitride,fluorine-doped silicate glass (FSG), a low-k dielectric, combinationsthereof, and/or other suitable materials. In various examples, thedielectric layer may be deposited by a CVD process, a subatmospheric CVD(SACVD) process, a flowable CVD process, an ALD process, a physicalvapor deposition (PVD) process, spin-on coating, and/or other suitableprocess. The deposited dielectric material is then thinned andplanarized, for example by a chemical mechanical polishing (CMP)process. The planarized dielectric layer is further recessed by a dryetching process, a wet etching process, and/or a combination thereof toform the isolation feature 203. As shown in FIG. 3 , the stack portions12S of the fin-shaped structures 212 rise above the isolation feature203.

Referring to FIGS. 1 and 3 , method 100 includes a block 108 where acladding layer 214 is formed over the first fin-shaped structure 212-1,the second fin-shaped structure 212-2, the third fin-shaped structure212-3, the fourth fin-shaped structure 212-4, and the fifth fin-shapedstructure 212-5. In some embodiments, the cladding layer 214 may have acomposition similar to that of the sacrificial layers 206. In oneexample, the cladding layer 214 may be formed of silicon germanium(SiGe). The common composition allows selective removal of thesacrificial layers 206 and the cladding layer 214 during the release ofchannel layers 208 in a subsequent process. In some other embodiments,while both the sacrificial layers 206 and the cladding layer 214 areformed of silicon germanium (SiGe), the sacrificial layers 206 and thecladding layer 214 may have different germanium contents to introducedifferent etch selectivity during formation of inner spacer recesses.The cladding layer 214 may have a germanium content smaller than that ofthe sacrificial layers 206. In some instances, the sacrificial layer 206may have a germanium content between about 20% and about 25% and thecladding layer 214 may have a germanium content between about 15% andabout 19%. At block 108, the cladding layer 214 may be epitaxially grownusing vapor phase epitaxy (VPE) or molecular bean epitaxy (MBE). In someimplementations not explicitly shown in FIG. 3 , the formation of thecladding layer 214 may be selective to the surfaces of the stackportions 12S of the fin-shaped structures 212 and little or no claddinglayer 214 may be deposited over the hard mask layer 210 or the isolationfeature 203. In some alternative implementations represented in FIG. 3 ,the formation of the cladding layer 214 may be conformal on thefin-shaped structures 212, including over the hard mask layer 210. Insome embodiments, operations at block 108 may include etch backprocesses to remove cladding layer 214 on the isolation feature 203. Anexample etch back process may be a dry etch process that includes use ofplasma of hydrogen bromide (HBr), oxygen (O₂), chlorine (Cl₂), ormixtures thereof.

Referring to FIGS. 1 and 4 , method 100 includes a block 110 where firstdummy fins 216 and second dummy fins 218 are formed. As shown in FIG. 4, the first dummy fins 216 dummy fins that fill the first spacings S1between the first fin-shaped structure 212-1 and the second fin-shapedstructure 212-2, between the third fin-shaped structure 212-3 and thefourth fin-shaped structure 212-4, and between the fourth fin-shapedstructure 212-4 and the fifth fin-shaped structure 212-5. The seconddummy fins 218 are dummy fins that fill the second spacings S2 betweenthe second fin-shaped structure 212-2 and the third fin-shaped structure212-3 and between other two similarly situated fin-shaped structures. Inan example process, a first dielectric layer 220 is first conformallydeposited over the workpiece 200, including along sidewalls of thefin-shaped structures 212 and the top surfaces of the isolation feature203. In some embodiments, the first dielectric layer 220 may includesilicon carbonitride (SiCN) or silicon oxycarbonitride (SiOCN) and maybe deposited using CVD or atomic layer deposition (ALD). As shown inFIG. 4 , the conformally deposited first dielectric layer 220 does notcompletely fill the first spacings S1 but may completely fill the secondspacings S2. In some instances where the first dielectric layer 220prematurely closes the opening between the second fin-shaped structure212-2 and the third fin-shaped structure 212-3, a void 219 may beobserved in the second dummy fin 218. After the deposition of the firstdielectric layer 220, a second dielectric layer 222 is deposited overthe workpiece 200, including over the first dielectric layer 220. Insome embodiments, the second dielectric layer 222 may include siliconoxide and may be deposited using spin-on coating, a flowable CVD processor a suitable deposition process. In some instances, in order to improveintegrity and density of the second dielectric layer 222, an annealprocess may be performed to anneal the second dielectric layer 222.After the deposition of the second dielectric layer 222, a planarizationprocess, such as an CMP process, may be performed to planarize the topsurfaces of the first dielectric layer 220 and the second dielectriclayer 222. After the planarization process, the first dummy fins 216 andthe second dummy fins 218 are formed.

Reference is still made to FIG. 4 . Because the first dummy fins 216 areformed in the first spacings S1, each of the first dummy fins 216 has awidth comparable to the first spacing S1 along the X direction. In oneembodiment, the width of the first dummy fin 216 is identical to thefirst spacing S1. Similarly, because the second dummy fins 218 areformed in the second spacings S2, each of the second dummy fins 218 hasa width comparable to the second spacing S2 along the X direction. Inone embodiment, the width of the second dummy fin 218 is identical tothe second spacing S2. At this stage, the first dummy fin 216 may beregarded as having the first dielectric layer 220 as an outer dielectriclayer 220 and the second dielectric layer 222 as an inner dielectriclayer 222. As illustrated in FIG. 4 , the outer dielectric layer 220wraps around the sidewalls and bottom surface of the inner dielectriclayer 222 and isolates the inner dielectric layer 222 from the claddinglayer 214. The second dummy fin 218 includes the first dielectric layer220 and is free of the second dielectric layer 222. Due to the differentconstructions, the first dummy fins 216 and the second dummy fins 218may have different etch resistance and properties. The first dielectriclayer 220, which may be formed of silicon carbonitride or siliconoxycarbonitride, is more etch resistant than the second dielectric layer222, which may be formed of silicon oxide. Because the second dummy fins218 are formed of the first dielectric layer 220 and free of the seconddielectric layer 222, the second dummy fins 218 may etch at a slowerrate than the first dummy fins 216 in an etch back process, such as theetch back process at block 112, as described below.

Referring to FIGS. 1 and 5 , method 100 includes a block 112 where thefirst dummy fins 216 and the second dummy fins 218 are etched back. Atblock 112, the first dummy fins 216 and the second dummy fins 218 may beselectively and anisotropically etched back to form first recesses 21and the second recesses 22. As shown in FIG. 5 , due to the differentconstructions of the first dummy fins 216 and the second dummy fins 218,the first recesses 21 are deeper than the second recesses 22. In someimplementations, the first recesses 21 may each have a first depth D1along the Z direction and the second recesses 22 may each have a seconddepth D2 along the Z direction. The first depth D1 is greater than thesecond depth D2. The etch back at block 112 may be performed using a dryetch process. An example dry etch process may include use of anoxygen-containing gas, hydrogen, a fluorine-containing gas (e.g., CF₄,SF₆, CH₂F₂, CHF₃, and/or C₂F₆), a chlorine-containing gas (e.g., Cl₂,CHCl₃, CCl₄, and/or BCl₃), a bromine-containing gas (e.g., HBr and/orCHBR₃), an iodine-containing gas, other suitable gases and/or plasmas,and/or combinations thereof.

Referring to FIGS. 1, 6 and 7 , method 100 includes a block 114 wherethe second dummy fins 218 are selectively trimmed. Because a cappinglayer 228 (to be described below) may be deposited in the recesses overthe first dummy fins 216 and the second dummy fins 218, the smallersecond depth D2 may result in a thinner capping layer 228 over thesecond dummy fins 218. Because the capping layer 228 functions as anetch-resistant protective layer, a smaller thickness thereof may causeundesirable removal of the capping layer 228 and excessive heightreduction of the second dummy fins 218. With a smaller height, thesecond dummy fins 218 may not separate source/drain features ofneighboring devices. To address this challenge, the present disclosureimplements block 114 to selectively trim the second dummy fins 218 toincrease the depth of the second recess 22 to form the deeper thirdrecess 23 (shown in FIG. 7 ).

Reference is first made to FIG. 6 . In an example process, the selectivetrimming at block 114 may include formation of a photoresist mask 224that exposes the second dummy fins 218 and covers the rest of theworkpiece 200. For example, a photoresist layer is first coated over theworkpiece 200 using spin-on coating or a suitable process. To patternthe photoresist layer to form the photoresist mask 224, the photoresistlayer is soft-baked, exposed to radiation transmitting through orreflected from a photomask, baked in a post-exposure bake process,developed in a developer solution, rinsed and dried. After thepatterning, the photoresist mask 224 includes an opening 226 thatexposes the second dummy fins 218. With the photoresist mask 224 inplace, he second dummy fins 218 may be etched in a dry etch process, awet etch process, or a suitable etch process. A suitable dry etchprocess may include use of use of an oxygen-containing gas, hydrogen, afluorine-containing gas (e.g., CF₄, SF₆, CH₂F₂, CHF₃, and/or C₂F₆), achlorine-containing gas (e.g., Cl₂, CHCl₃, CCl₄, and/or BCl₃), abromine-containing gas (e.g., HBr and/or CHBR₃), an iodine-containinggas, other suitable gases and/or plasmas, and/or combinations thereof. Asuitable wet etch process may include use of diluted hydrofluoric (DHF)acid or buffered hydrofluoric (BHF) acid. In some embodimentsillustrated in FIG. 7 , the trimming operations at block 114 mayincrease the second depth D2 of the second recess 22 to form a thirdrecess 23. In alternative embodiments, although the depth of the thirdrecess 23 is greater than the second depth D2, it may be smaller orgreater than the first depth D1. That is, the present disclosure doesnot require the third recess 23 to have a thickness identical to thefirst depth D1 as long as it is greater than the second depth D2. Asshown in FIG. 7 , after the selective trimming of the second dummy fins218, the photoresist mask 224 may be removed by ashing or a suitableprocess.

Referring to FIGS. 1 and 8 , method 100 includes a block 116 where acapping layer 228 is deposited over the first dummy fins 216 and thesecond dummy fins 218. In some embodiments, the capping layer 228 mayinclude a high-k dielectric material, such as a metal oxide. As usedherein, a high-k dielectric material refers to a dielectric materialthat has a dielectric constant greater than that of silicon dioxide(−3.9). Suitable metal oxide may include hafnium oxide, zirconium oxide,titanium oxide, tantalum oxide, or aluminum oxide. In one embodiment,the capping layer 228 includes hafnium oxide. In some implementations,the capping layer 228 may be deposited using CVD, flowable CVD, or asuitable deposition method. The capping layer 228, as deposited, maycover top surfaces of the first dummy fins 216, the second dummy fins218, the hard mask layer 210, and the cladding layer 214. The depositionof the capping layer 228 may be followed by a planarization process,such as a CMP process, to remove the capping layer 228 over thefin-shaped structures 212. At this point, the top surfaces of the firstdummy fins 216 and the second dummy fins 218 are coplanar. After theplanarization process, each of the first dummy fins 216 and the seconddummy fins 218 is capped by the capping layer 228. The capping layer 228may be regarded as a part of the first dummy fin 216 or the second dummyfin 218. In this regard, each of the second dummy fins 218 may beregarded as having a bottom portion formed of the first dielectric layer220 and the capping layer 228 over the bottom portion. That is, afteroperations at block 116, each of the first dummy fins 216 includes theouter dielectric layer 220, the inner dielectric layer 222, and thecapping layer 228; and each of the second dummy fins 218 includes thecapping layer 228 disposed on the first dielectric layer 220. In someinstances, the first dummy fins 216 may also be referred to as firsthybrid fins 216 or first dielectric fins 216. Similarly, the seconddummy fins 218 may also be referred to as second hybrid fins 218 orsecond dielectric fins 218. Because the first dummy fins 216 and thesecond dummy fins 218 only function as separation structures and are notformed of semiconductor materials, they do not form part of thefunctional circuit of the semiconductor device 200.

Referring to FIGS. 1 and 9 , method 100 includes a block 118 where oneor more dummy gate stacks 230 is formed over the fin-shaped structures212, the first dummy fins 216, and the second dummy fins 218. In someembodiments, a gate replacement process (or gate-last process) isadopted where the one or more dummy gate stacks 230 serves asplaceholders for functional gate structures. Other processes andconfiguration are possible. As the one-more dummy gate stacks 230appears out of the planes of the cross-sectional view in FIG. 9 , dottedlines are used to schematically show the relative position of the one ormore dummy gate stacks 230. Although the one or more dummy gate stacks230 are shown as a continuous structure that extends lengthwise alongthe X direction across the fin-shaped structures 212, they may includemore than one dummy gate segment. The regions of the fin-shapedstructures 212 underlying the one or more dummy gate stacks 230 may bereferred to as channel regions. Each of the channel regions in afin-shaped structure is sandwiched between two source/drain regions forsource/drain formation.

Each of the one or more dummy gate stacks 230 may include a dummydielectric layer and a dummy gate electrode. In some embodiments, theone or more dummy gate stacks 230 may be formed by various process stepssuch as layer deposition, patterning, etching, as well as other suitableprocessing steps. Exemplary layer deposition processes includelow-pressure CVD, CVD, plasma-enhanced CVD (PECVD), PVD, ALD, thermaloxidation, e-beam evaporation, or other suitable deposition techniques,or combinations thereof. The patterning process may include alithography process (e.g., photolithography or e-beam lithography) whichmay further include photoresist coating (e.g., spin-on coating), softbaking, mask aligning, exposure, post-exposure baking, photoresistdeveloping, rinsing, drying (e.g., spin-drying and/or hard baking),other suitable lithography techniques, and/or combinations thereof. Insome embodiments, the etching process may include dry etching (e.g., RIEetching), wet etching, and/or other etching methods. In an exampleprocess, the dummy dielectric layer, the dummy electrode layer, and agate-top hard mask layer are sequentially deposited over the workpiece200, including over the fin-shaped structures 212. The dummy dielectriclayer and the dummy electrode layer are then patterned usingphotolithography processes to form the one or more dummy gate stacks230. In some embodiments, the dummy dielectric layer may include siliconoxide and the dummy electrode layer may include polycrystalline silicon(polysilicon). The gate-top hard mask layer may include a silicon oxidelayer and a nitride layer.

Although not explicitly shown, after the formation of the one or moredummy gate stacks 230, one or more gate spacers may be deposited alongsidewalls of the one or more dummy gate stacks 230. The one or more gatespacers may include dielectric materials that allow selective removal ofthe one or more dummy gate stacks 230. Suitable dielectric materials forthe one or more gate spacers may include silicon nitride, siliconoxycarbonitride, silicon carbonitride, silicon oxide, siliconoxycarbide, silicon carbide, silico oxynitride, and/or combinationsthereof. In an example process, layers of the one or more gate spacersare first conformally deposited using CVD, subatmospheric CVD (SACVD),or ALD over the workpiece 200, including the one or more dummy gatestacks 230. An etch back process is then etched back to remove theselayers from top-facing surfaces, leaving behind the one or more gatespacers along sidewalls of the one or more dummy gate stacks 230.

Referring to FIGS. 1 and 10 , method 100 includes a block 120 where thesource/drain regions of the first fin-shaped structure 212-1, the secondfin-shaped structure 212-2, the third fin-shaped structure 212-3, thefourth fin-shaped structure 212-4, and the fifth fin-shaped structure212-5 are recessed. With the one or more dummy gate stacks 230 and theone or more gate spacers masking the channel regions, source/drainregions of the fin-shaped structures 212 are recessed to form a firstsource/drain trench 234-1, a second source/drain trench 234-2, a thirdsource/drain trench 234-3, a fourth source/drain trench 234-4, and afifth source/drain trench 234-5. The first source/drain trench 234-1 isdisposed in the source/drain region of the first fin-shaped structure212-1. The second source/drain trench 234-2 is disposed in thesource/drain region of the second fin-shaped structure 212-2. The thirdsource/drain trench 234-3 is disposed in the source/drain region of thethird fin-shaped structure 212-3. The fourth source/drain trench 234-4is disposed in the source/drain region of the fourth fin-shapedstructure 212-4. The fifth source/drain trench 234-5 is disposed in thesource/drain region of the fifth fin-shaped structure 212-5. For ease ofreference, the first source/drain trench 234-1, the second source/draintrench 234-2, the third source/drain trench 234-3, the fourthsource/drain trench 234-4, and the fifth source/drain trench 234-5 maybe collectively referred to as source/drain trenches 234. In someembodiments as illustrated in FIG. 10 , operations at block 120 maysubstantially remove the stack portions 12S of fin-shaped structures212. The recess at block 120 may include a dry etch process or asuitable etch process. For example, the dry etch process may implementan oxygen-containing gas, hydrogen, a fluorine-containing gas (e.g.,CF₄, SF₆, CH₂F₂, CHF₃, and/or C₂F₆), a chlorine-containing gas (e.g.,Cl₂, CHCl₃, CCl₄, and/or BCl₃), a bromine-containing gas (e.g., HBrand/or CHBR₃), an iodine-containing gas, other suitable gases and/orplasmas, and/or combinations thereof. As shown in FIG. 10 , sidewalls ofthe sacrificial layers 206 and the channel layers 208 in the channelregions are exposed in the source/drain trenches 234. Because thechannel regions are outside of cross-sectional plane in FIG. 10 , thesacrificial layers 206 and the channel layers 208 are illustrated indotted lines.

Referring to FIGS. 1 and 11 , method 100 includes a block 122 whereinner spacer features 232 are formed. At block 122, the sacrificiallayers 206 exposed in the source/drain trenches 234 are selectively andpartially recessed to form inner spacer recesses, while the exposedchannel layers 208 are substantially unetched. In an embodiment wherethe channel layers 208 consist essentially of silicon (S1) andsacrificial layers 206 consist essentially of silicon germanium (SiGe),the selective and partial recess of the sacrificial layers 206 mayinclude a SiGe oxidation process followed by a SiGe oxide removal. Inthat embodiments, the SiGe oxidation process may include use of ozone.In some other embodiments, the selective recess may be a selectiveisotropic etching process (e.g., a selective dry etching process or aselective wet etching process), and the extent at which the sacrificiallayers 206 are recessed is controlled by duration of the etchingprocess. The selective dry etching process may include use of one ormore fluorine-based etchants, such as fluorine gas orhydrofluorocarbons. The selective wet etching process may include ahydro fluoride (HF) or NH₄OH etchant. An inner spacer material layer isthen conformally deposited using CVD or ALD over the workpiece 200,including over and into the inner spacer recesses. The inner spacermaterial may include silicon nitride, silicon oxycarbonitride, siliconcarbonitride, silicon oxide, silicon oxycarbide, silicon carbide, orsilico oxynitride. After the deposition of the inner spacer materiallayer, the inner spacer material layer is etched back to form innerspacer features 232.

Referring to FIGS. 1 and 12 , method 100 includes a block 124 wherefirst-type source/drain features 236 and second-type source/drainfeatures 238 are formed in the source/drain trenches 234. In someembodiments, the first-type source/drain features 236 are n-typesource/drain features and the second-type source/drain features 238 arep-type source/drain features. In some alternative embodiments, thefirst-type source/drain features 236 are p-type source/drain featuresand the second-type source/drain features 238 are n-type source/drainfeatures. Because the first-type source/drain features 236 are differentfrom the second-type source/drain features 238, they may be formedseparately using at least a masking layer. In some embodiments, thefirst-type source/drain features 236 and the second-type source/drainfeatures 238 may be formed using an epitaxial process, such asvapor-phase epitaxy (VPE), ultra-high vacuum CVD (UHV-CVD), molecularbeam epitaxy (MBE), and/or other suitable processes. The epitaxialgrowth process may utilize gaseous and/or liquid precursors, whichinteract with the composition of the substrate 202 as well as thechannel layers 208. Example n-type source/drain features may includesilicon (Si), gallium arsenide (GaAs), arsenic-doped silicon (SiAs),phosphorus-doped silicon (SiP), phosphorus-doped gallium arsenide(GaAsP), or other suitable material. N-type source/drain features may bein-situ doped during the epitaxial process by introducing an n-typedopant, such as phosphorus (P), arsenic (As), or both, or by animplantation process (i.e., a junction implant process). Example p-typesource/drain features may include geranium (Ge), silicon germanium(SiGe), aluminum-doped gallium arsenide (AlGaAs), boron-doped silicongermanium (SiGeB), or other suitable material. P-type source/drainfeatures may be in-situ doped during the epitaxial process byintroducing a p-type dopant, such as boron (B), or by an implantationprocess (i.e., a junction implant process).

Referring to FIG. 1 , method 100 includes a block 126 where furtherprocesses are performed. Referring to FIGS. 1 and 14 , method 100 mayinclude further processes. Such further processes may include, forexample, deposition of a contact etch stop layer (CESL), deposition ofan interlayer dielectric (ILD) layer, removal of the dummy gate stacks230 (shown in FIG. 12 ), selective removal of the sacrificial layers 206in the channel regions, and formation of gate structures. In an exampleprocess, the CESL is first deposited over the workpiece 200. The CESLmay include silicon nitride, silicon oxide, silicon oxynitride, and/orother materials known in the art. The CESL may be deposited using ALD,plasma-enhanced chemical vapor deposition (PECVD) process and/or othersuitable deposition or oxidation processes. The ILD layer is depositedover the CESL. In some embodiments, the ILD layer includes materialssuch as tetraethylorthosilicate (TEOS) oxide, un-doped silicate glass,or doped silicon oxide such as borophosphosilicate glass (BPSG), fusedsilica glass (FSG), phosphosilicate glass (PSG), boron doped siliconglass (BSG), and/or other suitable dielectric materials. The ILD layermay be deposited by spin-on coating, a PECVD process or other suitabledeposition technique. In some embodiments, after formation of the ILDlayer, the workpiece 200 may be annealed to improve integrity of the ILDlayer. To remove excess materials and to expose top surfaces of thedummy gate stacks 230, a planarization process, such a chemicalmechanical polishing (CMP) process may be performed. The exposed dummygate stacks 230 are then removed from the workpiece 200. The removal ofthe dummy gate stacks 230 results in gate trenches over the channelregions defined by the one or more gate spacers. The removal of thedummy gate stacks 230 may include one or more etching processes that areselective to the material in the dummy gate stacks 230. For example, theremoval of the dummy gate stacks 230 may be performed using as aselective wet etch, a selective dry etch, or a combination thereof.After the removal of the dummy gate stacks 230, sidewalls of thecladding layer 214, channel layers 208 and sacrificial layers 206 in thechannel regions are exposed in the gate trenches.

After the removal of the dummy gate stacks 230, the sacrificial layers206 between the channel layers 208 and the cladding layer in the channelregions may be selectively removed to release the channel layers 208 toform channel members. The selective removal of the sacrificial layers206 may be implemented by selective dry etch, selective wet etch, orother selective etch processes. In some embodiments, the selective wetetching includes an APM etch (e.g., ammonia hydroxide-hydrogenperoxide-water mixture). In embodiments where the sacrificial layers 206and the cladding layer 214 are formed of silicon germanium, theselective removal includes silicon germanium oxidation followed by asilicon germanium oxide removal. For example, the oxidation may beprovided by ozone clean and then silicon germanium oxide removed by anetchant such as NH₄OH. Gate structures are then deposited into the gatetrenches to wrap around each of the channel members on the X-Z plane. Insome embodiments, the gate structure includes a gate dielectric layerand a gate electrode formed over the gate dielectric layer. In someembodiments, the gate dielectric layer may include an interfacial layerand a high-k dielectric layer. High-K gate dielectrics, as used anddescribed herein, include dielectric materials having a high dielectricconstant, for example, greater than that of thermal silicon oxide(−3.9). The interfacial layer may include a dielectric material such assilicon oxide, hafnium silicate, or silicon oxynitride. The interfaciallayer may be deposited using chemical oxidation, thermal oxidation,atomic layer deposition (ALD), chemical vapor deposition (CVD), and/orother suitable method. The high-K dielectric layer may include a high-Kdielectric layer such as hafnium oxide. Alternatively, the high-Kdielectric layer may include other high-K dielectrics, such as hafniumoxide (HfO), titanium oxide (TiO₂), hafnium zirconium oxide (HfZrO),tantalum oxide (Ta₂O₅), hafnium silicon oxide (HfSiO₄), zirconium oxide(ZrO₂), zirconium silicon oxide (ZrSiO₂), lanthanum oxide (La₂O₃),aluminum oxide (Al₂O₃), zirconium oxide (ZrO), yttrium oxide (Y₂O₃),SrTiO₃ (STO), BaTiO₃ (BTO), BaZrO, hafnium lanthanum oxide (HfLaO),lanthanum silicon oxide (LaSiO), aluminum silicon oxide (AlSiO), hafniumtantalum oxide (HfTaO), hafnium titanium oxide (HfTiO), (Ba,Sr)TiO₃(BST), silicon nitride (SiN), silicon oxynitride (SiON), combinationsthereof, or other suitable material. The high-K dielectric layer may beformed by ALD, physical vapor deposition (PVD), CVD, oxidation, and/orother suitable methods.

The gate electrode of the gate structure may include a single layer oralternatively a multi-layer structure, such as various combinations of ametal layer with a selected work function to enhance the deviceperformance (work function metal layer), a liner layer, a wetting layer,an adhesion layer, a metal alloy or a metal silicide. By way of example,the gate electrode may titanium nitride (TiN), titanium aluminum (TiAl),titanium aluminum nitride (TiAlN), tantalum nitride (TaN), tantalumaluminum (TaAl), tantalum aluminum nitride (TaAlN), tantalum aluminumcarbide (TaAlC), tantalum carbonitride (TaCN), aluminum (Al), tungsten(W), nickel (Ni), titanium (Ti), ruthenium (Ru), cobalt (Co), platinum(Pt), tantalum carbide (TaC), tantalum silicon nitride (TaSiN), copper(Cu), other refractory metals, or other suitable metal materials or acombination thereof. In various embodiments, the gate electrode of thegate structure may be formed by ALD, PVD, CVD, e-beam evaporation, orother suitable process. In various embodiments, a planarization process,such as a CMP process, may be performed to remove excessive materials toprovide a substantially planar top surface of the gate structures.

Reference is made to FIG. 3 . According to the present disclosure, thesecond spacing S2 is smaller than the first spacing S1 and the secondsmaller spacing S2 is a spacing for transistors of the same conductivitytypes. In FIG. 3 , the smaller spacing S2 is X-direction spacing betweenthe second fin-shaped structure 212-2 and the third fin-shaped structure212-3. The second fin-shaped structure 212-2 and the third fin-shapedstructure 212-3 are for transistors of one conductivity type and thefirst fin-shaped structure 212-1, the fourth fin-shaped structure 212-4and the fifth fin-shaped structure 212-5 are for transistors of theother conductivity types. For example, the second fin-shaped structure212-2 and the third fin-shaped structure 212-3 are for p-type MBCtransistors and the first fin-shaped structure 212-1, the fourthfin-shaped structure 212-4 and the fifth fin-shaped structure 212-5 arefor n-type MBC transistors. For another example, the second fin-shapedstructure 212-2 and the third fin-shaped structure 212-3 are for n-typeMBC transistors and the first fin-shaped structure 212-1, the fourthfin-shaped structure 212-4 and the fifth fin-shaped structure 212-5 arefor p-type MBC transistors. There are reasons for this arrangement. Forinstance, source/drain features for different types of MBC transistorsmay be formed separately and merging of different source/drain featuresmay be unlikely.

The smaller second spacing S2 provides benefits. In a standard cell witha plurality of n-type MBC transistors and a plurality of p-type MBCtransistors, reducing the spacing of neighboring n-type MBC transistorsor neighboring p-type MBC transistors may reduce the X-directiondimension of the standard cell for greater packing density. When theX-direction dimension of a standard cell remains fixed, the smallersecond spacing S2 between one type of devices may allow wider channelmembers for the other type of devices. The benefits of the presentdisclosure may be demonstrated in FIG. 13 , which illustrates a top viewof the semiconductor device 200. As shown in FIG. 13 , the semiconductordevice 200 includes a static random access memory (SRAM) cell 250. TheSRAM cell 250 includes a first pull-down transistor (PD-1), a secondpull-down transistor (PD-2), a first pull-up transistor (PU-1), a secondpull-up transistor (PU-2), a first pass-gate transistor (PG-1), and asecond pass-gate transistor (PG-2). In the implementations shown in FIG.13 , the SRAM cell 250 may further include a first isolation transistor(IS-1) and a second isolation transistor (IS-2). The first pass-gatetransistor (PG-1) is controlled by the first gate structure 240. Thefirst pull-down transistor (PD-1), the first pull-up transistor (PU-1),and the second isolation transistor (IS-2) may share a second gatestructure 242. The first isolation transistor (IS-1), the second pull-uptransistor (PU-2) and the second pull-down transistor (PD-2) may share athird gate structure 244. The second pass-gate transistor (PG-2) iscontrolled by a fourth gate structure 246.

In some embodiments, the first pull-down transistor (PD-1), the firstpass-gate transistor (PG-1), the second pass-gate transistor (PG-2), andthe second pull-down transistor (PD-2) are n-type MBC transistorsdisposed over p-type wells while the first pull-up transistor (PU-1) andthe second pull-up transistor (PU-2) are p-type devices disposed overn-type wells. The source/drain features of the first pull-downtransistor (PD-1), the first pass-gate transistor (PG-1), the secondpass-gate transistor (PG-2), and the second pull-down transistor (PD-2)are first-type source/drain features 236. The first pull-up transistor(PU-1) and the second pull-up transistor (PU-2) are second-typesource/drain features 238. In these embodiments, the first-typesource/drain features 236 are n-type source/drain features and thesecond-type source/drain features 238 are p-type source/drain features.The first-type source/drain features 236 are spaced apart from thesecond-type source/drain features 238 by the first dummy fins 216.Neighboring second-type source/drain features 238 are spaced apart fromone another by the second dummy fins 218. Neighboring-type are spacedapart from one another by the first dummy fins 216. As illustrated inFIG. 13 , the smaller second spacing S2 may allow dimensional reductionof the SRAM cell 250 or may allow channel width increase along the Xdirection for the n-type transistors (including the first pull-downtransistor (PD-1), the first pass-gate transistor (PG-1), the secondpass-gate transistor (PG-2), and the second pull-down transistor(PD-2)). In the latter case, increasing the channel widths of the n-typeMBC transistors may improve performance of the n-type MBC transistorsand reduce the minimum supply voltage (Vccmin) of the SRAM cell 250.

In one exemplary aspect, the present disclosure is directed to asemiconductor device. The semiconductor device includes a firstsource/drain feature, a second source/drain feature, a thirdsource/drain feature, a first dummy fin disposed between the firstsource/drain feature and the second source/drain feature along adirection to isolate the first source/drain feature from the secondsource/drain feature, and a second dummy fin disposed between the secondsource/drain feature and the third source/drain feature along thedirection to isolate the second source/drain feature from the thirdsource/drain feature. The first dummy fin includes an outer dielectriclayer, an inner dielectric layer over the outer dielectric layer, and afirst capping layer disposed over the outer dielectric layer and theinner dielectric layer. The second dummy fin includes a bottom portionand a second capping layer disposed over the bottom portion.

In some embodiments, the first source/drain feature is an n-typesource/drain feature and the second source/drain feature and the thirdsource/drain feature are p-type source/drain features. In someimplementations, the first source/drain feature includes silicon and ann-type dopant and the second source/drain feature and the thirdsource/drain feature include silicon germanium and a p-type dopant. Insome embodiments, the inner dielectric layer is spaced apart from thefirst source/drain feature and the second source/drain feature by theouter dielectric layer. In some instances, the outer dielectric layerincludes silicon carbonitride or silicon oxycarbonitride, the innerdielectric layer includes silicon oxide, and the first capping layerincludes hafnium oxide, zirconium oxide, titanium oxide, tantalum oxide,or aluminum oxide. In some embodiments, the bottom portion includessilicon carbonitride or silicon oxycarbonitride and the second cappinglayer includes hafnium oxide, zirconium oxide, titanium oxide, tantalumoxide, or aluminum oxide. In some embodiments, a top surface of theinner dielectric layer and a top surface of the bottom portion aresubstantially coplanar. In some instances, a width of the first dummyfin along the direction is greater than a width of the second dummy finalong the direction.

In another exemplary aspect, the present disclosure is directed to astatic random access memory (SRAM) cell. The SRAM cell includes apull-down transistor including a first source/drain feature, a pull-uptransistor including a second source/drain feature, a first dummy finseparating the pull-down transistor and the pull-up transistor along adirection, and a second dummy fin adjacent the second source/drainfeature. The pull-up transistor is disposed between the first dummy finand the second dummy fin along the direction. The first dummy finincludes an outer dielectric layer, an inner dielectric layer over theouter dielectric layer, and a first capping layer disposed over theouter dielectric layer and the inner dielectric layer. The second dummyfin includes a bottom portion and a second capping layer disposed overthe bottom portion.

In some embodiments, the pull-down transistor includes an n-typetransistor and the pull-up transistor includes a p-type transistor. Insome embodiments, the first source/drain feature includes silicon and ann-type dopant and the second source/drain feature includes silicongermanium and a p-type dopant. In some implementations, the innerdielectric layer is spaced apart from the first source/drain feature andthe second source/drain feature by the outer dielectric layer. In someembodiments, the outer dielectric layer includes silicon carbonitride orsilicon oxycarbonitride, the inner dielectric layer includes siliconoxide, and the first capping layer includes hafnium oxide, zirconiumoxide, titanium oxide, tantalum oxide, or aluminum oxide. In someinstances, the bottom portion includes silicon carbonitride or siliconoxycarbonitride and the second capping layer includes hafnium oxide,zirconium oxide, titanium oxide, tantalum oxide, or aluminum oxide. Insome implementations, a width of the first dummy fin along the directionis greater than a width of the second dummy fin along the direction.

In yet another exemplary aspect, the present disclosure is directed to amethod. The method includes receiving a workpiece that includes a firstfin-shaped structure extending lengthwise along a first direction andhaving a first base portion and a first stack portion over the firstbase portion, a second fin-shaped structure extending lengthwise alongthe first direction and having a second base portion and a second stackportion over the second base portion, wherein the first fin-shapedstructure is spaced apart from the second fin-shaped structure by afirst spacing, and a third fin-shaped structure extending lengthwisealong the first direction and having a third base portion and a thirdstack portion over the third base portion, wherein the second fin-shapedstructure is spaced apart from the third fin-shaped structure by asecond spacing smaller than the first spacing. The method furtherincludes forming an isolation feature between the first base portion andthe second base portion, and between the second base portion and thethird base portion, conformally depositing a first dielectric layer overthe first stack portion, the second stack portion, and the third stackportion, and the isolation feature, depositing a second dielectric layerover the first dielectric layer, planarizing the workpiece to form afirst dummy fin between the first stack portion and the second stackportion and a second dummy fin between the second stack portion and thethird stack portion, etching back the first dummy fin and the seconddummy fin, selectively etching back the second dummy fin, after theselectively etching, depositing a third dielectric layer over theworkpiece, recessing the first stack portion, the second stack portion,and the third stack portion, and forming a first source/drain featureover the first base portion, a second source/drain feature over thesecond base portion, and a third source/drain feature over the thirdbase portion.

In some embodiments, each of the first stack portion, the second stackportion, and the third stack portion includes a plurality of channellayers interleaved by a plurality of sacrificial layer. The plurality ofchannel layers include silicon and the plurality of sacrificial layersinclude silicon germanium. In some embodiments, the method furtherincludes, before the conformally depositing, depositing a silicongermanium cladding layer over the first stack portion, the second stackportion, and the third stack portion. In some embodiments, the firstdielectric layer includes silicon carbonitride or siliconoxycarbonitride, the second dielectric layer includes silicon oxide, andthe third dielectric layer includes hafnium oxide, zirconium oxide,titanium oxide, tantalum oxide, or aluminum oxide. In some instances,after the etching back of the first dummy fin and the second dummy fin,a top surface of the second dummy fin is further away from the isolationfeature than a top surface of the first dummy fin.

The foregoing outlines features of several embodiments so that those ofordinary skill in the art may better understand the aspects of thepresent disclosure. Those of ordinary skill in the art should appreciatethat they may readily use the present disclosure as a basis fordesigning or modifying other processes and structures for carrying outthe same purposes and/or achieving the same advantages of theembodiments introduced herein. Those of ordinary skill in the art shouldalso realize that such equivalent constructions do not depart from thespirit and scope of the present disclosure, and that they may makevarious changes, substitutions, and alterations herein without departingfrom the spirit and scope of the present disclosure.

What is claimed is:
 1. A semiconductor device, comprising: a firstsource/drain feature; a second source/drain feature; a thirdsource/drain feature; a first dummy fin disposed between the firstsource/drain feature and the second source/drain feature along adirection to isolate the first source/drain feature from the secondsource/drain feature; a second dummy fin disposed between the secondsource/drain feature and the third source/drain feature along thedirection to isolate the second source/drain feature from the thirdsource/drain feature; and a third dummy fin disposed adjacent to thethird source/drain feature such that the second dummy fin and the thirddummy fin sandwich the third source/drain feature along the direction,wherein the first dummy fin comprises a first outer dielectric layer, afirst inner dielectric layer over the first outer dielectric layer, anda first capping layer disposed directly on top surfaces of the firstouter dielectric layer and the first inner dielectric layer, wherein thesecond dummy fin comprises a bottom portion and a second capping layerdisposed directly on a top surface of the bottom portion, wherein thethird dummy fin comprises a second outer dielectric layer, a secondinner dielectric layer over the second outer dielectric layer, and athird capping layer disposed directly on top surfaces of the secondouter dielectric layer and the second inner dielectric layer, whereinthe bottom portion comprises a uniform dielectric material and a voidcompletely and continuously surrounded by the uniform dielectricmaterial, and wherein the first source/drain feature is of a firstconductivity type, the second source/drain feature and the thirdsource/drain feature are of a second conductivity type different fromthe first conductivity type.
 2. The semiconductor device of claim 1,wherein the first source/drain feature is an n-type source/drainfeature, wherein the second source/drain feature and the thirdsource/drain feature are p-type source/drain features.
 3. Thesemiconductor device of claim 1, wherein the first source/drain featurecomprises silicon and an n-type dopant, wherein the second source/drainfeature and the third source/drain feature comprise silicon germaniumand a p-type dopant.
 4. The semiconductor device of claim 1, wherein thefirst inner dielectric layer is spaced apart from the first source/drainfeature and the second source/drain feature by the first outerdielectric layer, and the semiconductor device further comprises afourth source/drain feature adjacent to the third dummy fin, such thatthe third source/drain feature and the fourth source/drain featuresandwich the third dummy fin along the direction and are isolated by thethird dummy fin, wherein the fourth source/drain feature is of the firstconductivity type.
 5. The semiconductor device of claim 1, wherein thefirst outer dielectric layer and the second outer dielectric layercomprise silicon carbonitride or silicon oxycarbonitride, wherein thefirst inner dielectric layer and the second inner dielectric layercomprise silicon oxide, wherein the first capping layer and the thirdcapping layer comprise hafnium oxide, zirconium oxide, titanium oxide,tantalum oxide, or aluminum oxide.
 6. The semiconductor device of claim1, wherein the bottom portion comprises silicon carbonitride or siliconoxycarbonitride, wherein the second capping layer comprises hafniumoxide, zirconium oxide, titanium oxide, tantalum oxide, or aluminumoxide.
 7. The semiconductor device of claim 1, wherein a top surface ofthe first inner dielectric layer and a top surface of the bottom portionare substantially coplanar.
 8. The semiconductor device of claim 1,wherein a width of the first dummy fin along the direction is greaterthan a width of the second dummy fin along the direction.
 9. A staticrandom access memory (SRAM) cell, comprising: a pull-down transistorcomprising a first source/drain feature; a pull-up transistor comprisinga second source/drain feature; an isolation transistor comprising athird source/drain feature; a first dummy fin separating the pull-downtransistor and the pull-up transistor along a direction; and a seconddummy fin separating the second source/drain feature and the thirdsource/drain feature, wherein the pull-up transistor is disposed betweenthe first dummy fin and the second dummy fin along the direction,wherein the second source/drain feature is in physical contact with thefirst dummy fin and the second dummy fin, wherein the first dummy fincomprises an outer dielectric layer, an inner dielectric layer over theouter dielectric layer, and a first capping layer disposed directly ontop surfaces of the outer dielectric layer and the inner dielectriclayer, wherein the second dummy fin comprises a bottom portion and asecond capping layer disposed directly on a top surface of the bottomportion.
 10. The SRAM cell of claim 9, wherein the pull-down transistorcomprises an n-type transistor, wherein the pull-up transistor comprisesa p-type transistor, wherein the pull-down transistor, the pull-uptransistor, and the isolation transistor share a single gate structure.11. The SRAM cell of claim 9, wherein the first source/drain featurecomprises silicon and an n-type dopant, wherein the second source/drainfeature comprises silicon germanium and a p-type dopant.
 12. The SRAMcell of claim 9, further comprising a first isolation feature directlybelow the first dummy fin and a second isolation feature directly belowthe second dummy fin, wherein a bottom surface of the first dummy findirectly contacts a topmost surface of the first isolation feature, abottom surface of the second dummy fin directly contacts a topmostsurface of the second isolation feature, and wherein the innerdielectric layer is spaced apart from the first source/drain feature andthe second source/drain feature by the outer dielectric layer.
 13. TheSRAM cell of claim 9, wherein the outer dielectric layer comprisessilicon carbonitride or silicon oxycarbonitride, wherein the innerdielectric layer comprises silicon oxide, wherein the first cappinglayer comprises hafnium oxide, zirconium oxide, titanium oxide, tantalumoxide, or aluminum oxide.
 14. The SRAM cell of claim 9, wherein thebottom portion comprises a dielectric material and a void completelysurrounded by the dielectric material, wherein the bottom portioncomprises silicon carbonitride or silicon oxycarbonitride, wherein thesecond capping layer comprises hafnium oxide, zirconium oxide, titaniumoxide, tantalum oxide, or aluminum oxide.
 15. The SRAM cell of claim 9,wherein a width of the first dummy fin along the direction is greaterthan a width of the second dummy fin along the direction.
 16. Asemiconductor structure, comprising: a first source/drain feature; asecond source/drain feature; a third source/drain feature; a first dummyfin disposed between the first source/drain feature and the secondsource/drain feature along a direction; a second dummy fin disposedbetween the second source/drain feature and the third source/drainfeature along the direction; and a third dummy fin disposed adjacent tothe third source/drain feature along the direction, such that the thirdsource/drain feature is between the second dummy fin and the third dummyfin, wherein the first source/drain feature comprises silicon and ann-type dopant, wherein the second source/drain feature and the thirdsource/drain feature comprise silicon germanium and a p-type dopant,wherein both a first width of the first dummy fin along the directionand a third width of the third dummy fin along the direction are greaterthan a second width of the second dummy fin along the direction, whereina top surface of the first dummy fin, a top surface of the third dummyfin, and a top surface of the second dummy fin are coplanar, wherein thefirst dummy fin comprises a first dielectric layer as a first outerdielectric layer and a second dielectric layer as a first innerdielectric layer, wherein the second dummy fin comprises the firstdielectric layer as a bottom portion and is free of the seconddielectric layer, wherein the third dummy fin comprises the firstdielectric layer as a second outer dielectric layer and the seconddielectric layer as a second inner dielectric layer, and wherein thefirst dielectric layer is more etch resistant than the second dielectriclayer.
 17. The semiconductor structure of claim 16, further comprising:a vertical stack of bridge-like channel members in contact with thefirst source/drain features; and a plurality of inner spacer featuresinterleaving the vertical stack of bridge-like channel members.
 18. Thesemiconductor structure of claim 16, wherein the first dummy fin furthercomprises a first capping layer disposed directly on top surfaces of thefirst outer dielectric layer and the first inner dielectric layer,wherein the second dummy fin further comprises a second capping layerdisposed directly on a top surface of the bottom portion, and whereinthe third dummy fin further comprises a third capping layer disposeddirectly on top surfaces of the second outer dielectric layer and thesecond inner dielectric layer.
 19. The semiconductor structure of claim18, wherein the first outer dielectric layer and the second outerdielectric layer comprise silicon carbonitride or siliconoxycarbonitride, wherein the first inner dielectric layer and the secondinner dielectric layer comprise silicon oxide, wherein the first cappinglayer and the third capping layer comprise hafnium oxide, zirconiumoxide, titanium oxide, tantalum oxide, or aluminum oxide.
 20. Thesemiconductor structure of claim 18, wherein the bottom portioncomprises silicon carbonitride or silicon oxycarbonitride, wherein thesecond capping layer comprises hafnium oxide, zirconium oxide, titaniumoxide, tantalum oxide, or aluminum oxide.